Mr Johnson – who is due to hit the campaign trail in Scotland today – said 2020 under his hard-Left rival would see the “horror show” of two referendums – one on Brexit and one on breaking up the UK. He warned that voters could wake up on December 13 finding Mr Corbyn “at the head of his technicolour yawn of a coalition”, subjecting the country to “yet more dither and delay”. Mr Corbyn has been accused of opening the door to a backroom deal” with the SNP.

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He has said hat he would not give Miss Sturgeon the powers for another separation vote “any time soon”, but could do so “at a much later stage”.

Rebecca Long-Bailey, the shadow business secretary, said at the weekend that Labour would allow a referendum if the SNP and pro-independence Greens win a majority at the 2021 Holyrood election.

But Miss Sturgeon has insisted Mr Corbyn will capitulate over allowing a referendum next year as he is under “no illusions” about her demands for putting him in Downing Street.

Speaking ahead of his visit to the north-east of Scotland, Mr Johnson said it was clear the Labour leader “would spend next year dancing to the SNP’s tune”.

Nicola Sturgeon won’t accept the 2014 referendum was once in a generation vote (Image: Getty)

He said next’s month poll was “crucial” and the Scottish Conservatives could halt a second independence referendum and “get Brexit done”.

Mr Johnson added: “So we can spend 2020 taking back control of our fishing waters, getting a fairer deal for our farmers, and investing in public services to give people better healthcare and better education.

“The other choice is a vote for Jeremy Corbyn, who would spend next year dancing to the SNP’s tune, wasting the year with two divisive referendums – one on the EU and one to give up on our Union.”

The Tory leader, who has ruled out allowing a re-run of the 2014 referendum while he is PM, insisted only the Conservatives could stop the SNP plans to break up the UK.

“I will never give up on our incredible Union,” Mr Johnson pledged.

Earlier on the steps of No10 Mr Johnson urged voters to “come with us” and to get his “oven-ready” Brexit deal through Parliament.

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“Come with us, a Government that believes Britain should stand tall in the world,” he said. “Or go with Jeremy Corbyn and the Labour Party who sided with Vladimir Putin when Russia ordered poisonings on the streets of Salisbury.

“Come with us. Get Brexit done, and take this country forward. Or, and this is the alternative next year, spend the whole of 2020 in a horror show of yet more dither and delay.”

It came after he visited Buckingham Palace where the Queen agreed to formally dissolve Parliament.

Accusing Mr Corbyn of having “done a deal” with the Nationalists to “assist the break-up of the Union”, Mr Johnson added: “There is only one way to avoid that nightmare and that is to vote for a moderate and compassionate One Nation Conservative government.”

Asked yesterday if the price for supporting a Labour government would be an independence referendum, Miss Sturgeon said: “There is a really important principle at stake here.

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“It is absolutely legitimate for Jeremy Corbyn or anybody else to oppose independence, the principle is that it’s for the people of Scotland to decide if there is an independence referendum and when that is.

“I don’t think any Westminster politician looking for SNP support could expect to get that if they don’t respect that basic principle.”

The issue overshadowed Scottish Labour’s election launch in Maryhill, Glasgow, with leader Richard Leonard insisting: “There is no pact with the SNP, there’ll be no deals with the SNP, there’ll be no coalition with the SNP.”

He added: “If we are a minority Labour government, we would draft a Queen’s Speech and it would then be up to the SNP whether they voted for that progressive, transformative Queen’s Speech or not. If they did not vote for it, they would have to answer to the people of Scotland.”

Pressed on Miss Long-Bailey’s comments, he said that he planned on winning the 2021 Holyrood election and there will be no mandate.

But Scottish Tory chief whip Maurice Golden said: “Richard Leonard’s assertion that Labour will come anywhere close to winning in 2021 is delusional.

“The truth is, when Jeremy Corbyn and Nicola Sturgeon attempt to strike a pro-independence alliance, Richard Leonard will be powerless to stop it.”

The start of the Tories’ campaign was hit by the resignation of senior minister Alun Cairns over his links to a Conservative candidate accused of sabotaging a rape trial.

The Welsh Secretary said he was standing down “in light of continued speculation” about allegations relating to the “actions of a party employee and candidate for the Welsh Assembly elections in the Vale of Glamorgan”.

Mr Johnson said he was pleased that Mr Cairns was “fully” cooperating with a Cabinet Office probe.